The present invention relates to a spray deposition process and in particular to a metallurgical spray deposition process.
The process is also useful for producing thick and thin coatings and other sprayed metal deposits sprayed onto substrates of all kinds, particularly where some of the topographical features are difficult to cover or fill due to complex geometries.
It is known to use metallurgical spray deposition techniques for producing tools, moulds, dies and other bodies of significant thickness. Problems have been encountered in using such techniques from the point of view of inherent porosity in the spray deposited material and internal stresses which arise during the spray deposition processes. Attempts have been made to deal with these problems and known techniques are described in, for example, WO-A-96/09421 and PCT patent application GB97/00590.
In a wide variety of commercially important thermal spray processes, the manner in which sprayed droplets impinge, spread and solidify on deposition is critical in influencing the subsequent properties of the manufactured coating or deposit. The first droplets to be deposited will determine the properties at the coating/substrate interface. In the case of spray forming of free standing shapes for mould tooling, the first deposited droplets determine the accuracy of replication and tooling wear properties. As deposition continues, droplet deposition behaviour controls the bulk microstructure (such as volume fraction, morphology and size of porosity) consequently determining the coating or deposit bulk properties. At all stages of deposition, droplet rebounding or splashing leads to a reduction in process yield. Recent experimental evidence suggests that droplet splashing occurs to a significant and greater extent than previously believed.
A further problem with sprayforming onto patterns or substrates having an object surface of varied (three dimensional) topography arises due to the fact that metal spray processes are xe2x80x9cline of sightxe2x80x9d processes in which Is known effects of shadowing and bridging occur for certain configurations of object surface topography.
A further problem occurs at internal and external edges of object surface topography, where poor quality deposit integrity can occur resulting in poor quality integrity to the deposit. This can result in flaking or crumbling of the deposit at corners and edges.
An improved spray deposition process has now been devised. According to the invention there is provided a process for producing a metallic tool, mould, die or other body of significant thickness or a coating, the process comprising directing a spray comprising molten metallic droplets carried by a propelling gas toward an object surface of a substrate or pattern so as to build up a metallic deposit or coating comprising the mould, tool, die, body or coating on the object surface of the substrate or pattern, wherein at one or more predetermined stages during spraying droplets of a relatively large mean size are sprayed and at one or more other stages droplets of a relatively smaller mean size are sprayed.
The relatively larger droplets are preferably sprayed at a stage preceding the spraying of relatively smaller droplets. Desirably, where the substrate or pattern includes topographical relief features, the spraying of the relatively larger droplets is dependent upon the nature of and/or the location of the topographical relief features.
It is a preferred feature of the invention that when spraying over portions of the object surface comprising topographical-features with a depth/width aspect ratio for example, or into or onto sharp corners or the like, that metallic droplets of relatively large sizes are sprayed, at least initially, in order to reduce shadowing, bridging, and poorly defined edge or corner detail which has been found to be a problem with prior art techniques.
It has been found that for producing detail and reducing shadowing or bridging, metallic spray droplets of mean diameters of substantially 200 microns and above (preferably substantially 350 microns and above) produce highly beneficial results. This result is surprising because trends in spray deposition research and practices have herebefore tended to suggest that finely sprayed droplets and relatively higher droplet spray velocities should produce improved results during the manufacture of coatings and most other products when using spray deposition techniques. Smaller droplet sizes have also been preferred because deposit porosity is minimised by using smaller droplet sizes. This is one of the premises behind the development of techniques such as plasma spraying and high velocity oxy-fuel metallic spraying techniques.
Additionally, spraying of larger droplet sizes for initial deposition, (including for coatings) has been found to result in reduced porosity in the deposited material immediately adjacent the substrate or pattern.
It is preferred that the propelling gas of the spray is within a pressure range lower than that normally recommended for use with a particular metal spraying apparatus. The operating pressure will therefore be different for different equipment, but is preferably at or about 3 bar or less. This results in the relatively large droplets desired, and relatively low droplet velocities compared with known techniques.
It is preferred that the droplets are produced by arc spraying, because arc spraying typically produces coarser droplet sizes than other known spray deposition processes. Conventional arc spraying apparatus has however not been designed for spraying at the larger droplet sizes of the present invention and modification and frequent cleaning of arc spray guns during proving of the invention has been found to be necessary. It is believed that this is strongly indicative of the process of spraying metallic droplets at the dimensions preferred being novel and inventive per se.
Preferably relatively high currents are used in the arc spraying process compared with the currents used in conventional arc spraying techniques.
It is important to control internal stresses in relatively thick deposited bodies formed in metallurgical spray deposition processes. WO-A-96/09421 discloses metallurgical spray deposition techniques which may be used to control internal stresses in deposited bodies. The relatively large droplet sizes required to improve reproduction of detail and edge definition from the object surface of the substrate or pattern (and also to inhibit bridging or shadowing) have however been found not to be suitable for control of stresses. It is therefore preferred that in the process according to the invention, process parameters are varied such that the relatively larger droplets are sprayed during the period and over the areas where detail of the substrate is required to be reproduced. Relatively smaller droplets are sprayed after the period when the detail has been replicated as required, and in other portions of the deposit where it is not required to replicate detail, preferably under conditions to control internal stresses in the deposit.
Typically therefore, the process according to the invention may comprise initial spraying of relatively large droplets onto the object surface of the substrate or pattern where detail replication is required (such as, for example, edge definition is to be reproduced, and/or where shadowing is to be avoided), and then subsequent modification of the spray parameters, (preferably as soon as possible after the said pattern detail and edge definition have been achieved), so as to spray relatively smaller droplets onto the object surface of the pattern or substrate. (Preferably in order to bring stress control into operation, as described, for example, in WO-A96/09421).
Alternatively, the process may comprise spraying of relatively large droplets from one spray source onto the object surface of the substrate or pattern where detail is required (such as edge definition is to be reproduced, and/or shadowing effects are to be ameliorated), and introducing a further spray of relatively smaller metallic droplets from a second spray source (preferably concurrently with the first spray), the further spray preferably being tailored to minimise internal stresses in the deposit.
It is preferred that the control step c) is operated by control means (preferably computer control means) and pre-programmed.
To achieve the required control, one or more spray guns are preferably mounted on manipulator means, such as an industrial robot which is preferably programmed, advantageously together with the spray guns, by the control means. Alternatively, or additionally, the one or more sprays of metallic droplets generated by the spray guns may be scannable, in which case the means for scanning the sprays is preferably coordinated and controlled, preferably by the same control means.